MAINTAINERS: update cxgb4vf maintainer to Vishal

Casey Leedomn <leedom@chelsio.com> is bouncing,
Vishal indicated he's happy to take the role.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>